Tube bending mandrel



April 7, 1970 w. u. MATSON 3,504,520

} TUBE BENDING MANDREL Filed Feb. 26, 1968 FIG. 2

INVENTOR WILLIAM U. MA TSON BY W ATTORNEYS United States Patent O 3,504,520 TUBE BENDING MANDREL William U. Matson, Oxon Hill, Md., assignor to the United States of America as represented by the Secretary of the Navy Filed Feb. 26, 1968, Ser. No. 708,357 Int. Cl. B21d 9/00 U.S. Cl. 72-466 6 Claims ABSTRACT OF THE DISCLOSURE An articulated mandrel having a rigid stem portion and a flexible portion for making H-plane bends in waveguides. The flexible portion of the mandrel is comprised of a plurality of links, each having a substantially rectangular cross-section with curved end surfaces. The links are pivotally interconnected by disks having annular rims formed thereon for interlocking engagement with grooves in said links.

STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates to mandrels for use in bending hollow bodies, and more particularly to flexible mandrels for supporting the interior of a rectangular waveguide tube during the bending thereof. During the bending of metallic waveguide tubes, it is extremely important to maintain the original cross-sectional dimensions of the interior of the tube within very close tolerances so that electromagnetic waves may be propagated along the tube without distortion of the signal. Any deviations from designed crosssectional dimensions of the waveguide tube, such as wrinkling, buckling, scoring, etc., will result in undesirable distortion of the electromagnetic transmission characteristics of the tubes. Large tensile and compressive stresses are developed within the walls of the waveguide tube during the bending thereof, which stresses may cause buckling or collapsing of the walls at the place of the bend. In draw bending, the rectangular waveguide tube is drawn over a mandrel between a pressure die and a wiping die which exert very high pressures upon the external surfaces of the tubing, and a flexible mandrel is inserted into the interior of the tube to prevent buckling or collapsing of the walls of waveguide tube. The mandrel is intended to uniformly support the interior of the tube at all times during the bending of the tube and should be constructed of smooth hard surfaces to prevent scoring of the interior walls when the tube is drawn over the mandrel. Moreover, the mandrel should be flexible to permit bending of the tube as well as removal of the mandrel from the bent waveguide tube. Although prior art devices have experienced difliculty in preserving the interior dimensions of rectangular waveguide tube while bending the tube in the plane of the narrow side of the tube, termed Ebend, these difficulties become accentuated when the tube is bent in the plane parallel to the broad side thereof, while making an H bend. Previously known devices which have been available for making H-plane bends have not been able to consistently make such bends without scratching and scoring the interior walls of the tube or frequently permitting the walls of the tube to collapse under the high external pressures applied by the pressure die and the wiping die.

SUMMARY OF THE INVENTION The general purpose of this invention is to provide an 3,504,520 Patented Apr. 7, 1970 improved mandrel for making H-plane bends in rectangular waveguide tubing while maintaining the original internal cross-sectional dimensions of the tubing and preventing the tubing from becoming scored or having burrs or other surface irregularities formed therein during the bending operation. To attain this result, the present invention contemplates a mandrel having a rigid stem portion and a flexible body portion secured at one end thereof to the stem, the body portion having a plurality of links which are pivotally interconnected by means of disks interposed between adjacent links. The links are closely spaced together and have an external configuration similar to the interior configuration of the waveguide tube and the interlocked assembly of links and disks is so designed as to have sufiicient mechanical strength to withstand the pressing forces exerted upon the tube by the pressure die and the wiping die. The links are also so designed so as to present no sharp edges or surfaces to the interior of the waveguide tube which might otherwise score the tube during the bending operation. The design of the links and the interconnecting disks is such that the mandrel is capable of resisting pressing forces and also has great strength to prevent breaking or separation of the mandrel during the draw bend operation.

Therefore, it is an object of the present invention to provide an articulated mandrel for supporting the interior of rectangular waveguide tube during the bending thereof.

Another object of the present invention is to provide an articulated mandrel which will prevent scoring or scratching of the interior surfaces of a rectangular waveguide tube during the bending thereof.

A further object of the present invention is to provide a tube bending mandrel having great mechanical strength and being capable of preserving the interior cross-section dimensions of a rectangular waveguide tube during the bending of such tubes in the plane parallel to the broad side thereof.

Other objects and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 represents a side elevation of a mandrel constructed in accordance with the present invention;

FIG. 2 is a top plan view of the mandrel shown in FIG. 1;

FIG. 3 is an exploded view of one link and two of the interconnecting disks; and

FIG. 4 is a perspective view of the stem portion of the mandrel constructed in accordance with an alternative embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference numerals designate like parts throughout the various views, the mandrel of the present invention is shown as having a rigid stem portion 11 for the purpose of holding the flexible portion 12 within that section of the waveguide tubing which is to be bent. The flexible portion of the mandrel is comprised of a plurality of links 14 which are interconnected with one another by means of interlocking disks 15. Each disk has formed thereon a raised annular rim 16 which extends outwardly from opposite sides of the disk for interlocking engagement with corresponding grooves formed in the mandrel links.

Each link 14 is comprised of a body section 17 and a retaining plate 18. Each link body section is generally rectangular in configuration and has a fiat surface 19 on one side thereof while the end portions of the body sections are formed into arcuate segments 21 and 22, the thickness of the arcuate segments being greater than the thickness of the flat base plates of the link so that a portion of the arcuate segments extend outwardly from the base plate 19. The thickness or width of the arcuate segments should be substantially the same or slightly less than the internal width of the rectangular waveguide to the bent, and the thickness of the body portion 17 of the link would be approximately /2 of the internal width of the waveguide. The length of the link body 17 would be substantially the same as or slightly less than the inter nal cross-sectional length of the rectangular waveguide tube to be bent. The link body 17 has a first arcuate groove 23 cut therein to receive a portion of the rim 16 on one interconnecting disk and additionally has a second arcuate groove 24 formed therein to receive a portion of a second rim 16 on a second interconnecting disk while the disk retaining plate 18 has complementary grooves and 26 (not shown) formed in one side thereof to correspond with grooves 23 and 24 to also engage portions of the rims 16 on the interconnecting disks. With the retaining plate removed from the body of the link, first and second interconnecting disks may be placed with their rim portions received within the arcuate grooves 23 and 24 and then the retaining plate 18 may be placed over both interconnecting disks 15 so that their rim portions are received within the arcuate grooves 25 and 26 of the retaining plate. Thus, when the retaining plate is secured to the body of the link by means of countersunk screws 27 or other suitable means, the disks 15 are securely coupled to the links because the disk rims are captivated within the grooves formed in the link body and its retaining plate, however, the disks are permitted to rotate within the grooves to provide flexibility to a series of links interconnected in this manner.

The retaining plates 18 are provided with rounded or arcuate end portions 28 to be received closely within recess between the raised arcuate segments 21 and 22 on the link body and the thickness of the retaining plate is such that, when the retaining plate is received within the recess in the link body between the arcuate segments 21 and 22 and coupled to the link body, the outer surface 29 is flush with the edges 31 of the arcuate segments.

It will therefore be seen that when the flexible portion of the mandrel is assembled, each link will have a crosssection wherein the length and width is substantially the same as the internal cross-sectional dimensions of the Waveguide to be bent and that each link will also be a substantially solid body having extremely high strength and therefore will be capable of resisting the crushing forces exerted upon the external surfaces of the Waveguide by the pressure die and the wiping die. It will also be seen that adjacent links will be flexibly interconnected by means of a solid disk having integrally formed annular rims 16 and thus the mandrel is permitted to flex during the bending of the tube and yet has sufiicient strength to prevent breaking of either the links or the interconnecting disks under the tensile and shear stresses developed as the waveguide is drawn over the mandrel during the bending operation. The sides 32 of the link body and the sides 33 of the retaining plate are tapered to form two intersecting flat shoulders as shown in the drawings to permit relative pivotal flexing between adjacent links while at the same time providing a stop means for establishing a minimum arc of curvature of the flexible portion of the mandrel which occurs when the tapered flat shoulders of one link engage the corresponding shoulders of an adjacent link.

The stem or holder is a long rod having a rectangular cross-section corresponding substantially to the interior cross-section of the Waveguide tubing. One end of the holder is adapted to be secured to a portion of the bending machine to maintain the mandrel in a predetermined location within the work piece while the other end of the holder, which is adjacent to the flexible portion of the mandrel, has a fitting thereon for mechanically coupling the flexible link assembly to the holder. To provide this coupling, this end of the holder has a recess formed in one side thereof and being of such size and configuration as to closely receive one of the disk retaining plates 18. As previously described, the disk retaining plate has an annular recess formed therein to receive the rim portion of the disk and similarly the recess in the mandrel holder is also provided with a complementary arcuate groove to receive a portion of the disk rim 16, which grooves cooperate to mechanically interlock the link assembly with the mandrel stem in the same manner as the links are pivotally connected together in the manner previously described. The mandrel holder may be designed in two pieces, to provide a short portion 11, having the coupling formed thereon for connection with the flexible link assembly, and a long rectangular shaft having a cut away portion of reduced thickness at one end for connection to a smaller cut away section 13 on the short portion to provide an overlapping junction having the same cross-section as the remainder of the holder. Instead of designing the end of the draw bar to include a retaining plate 18 for purposes of engaging a disk 15 on the flexible link assembly, as shown in FIGS. 1 and 2, the draw bar may alternately be designed in accordance with the embodiment illustration in FIG. 4. In this embodiment, the terminal portion of the draw bar is machined in such a manner that the draw bar terminates in a connector 36 having a configuration representing one half of a connecting disk, with an annular rim 37 formed thereon, which disk and rim are similar in all respects to the interconnecting disks 15 used in the link assembly. This half disk 36 is then coupled within the arcuate grooves 23 and 24 in the first link while interconnecting disk 15 is received within the arcuate grooves 24 and 26 of the first link.

For optimum results in the production of waveguide bends, the parts of this mandrel should be constructed of polished hardened steel or other strong materials capable of withstanding the crushing forces exerted thereon externally by the pressure die and the wiping die. Also all edges or corners on the various parts and elements comprising this mandrel assembly should be slightly rounded and polished to further reduce the possibility of imparting scratches to the interior walls of the waveguide.

The design of this mandrel combines the advantages of high strength and rigidity which are necessary to produce accurate and smooth bends in a waveguide and this design further enables ease of assembly and disassembly so that the length of the flexible link assembly may be readily varied to lengthen or shorten the link assembly as required for producing bends in ditferent lengths of tube.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A mandrel for bending hollow members comprising:

a rigid stem,

at flexible body attached at one end thereof to said rigid stem,

said flexible body including a plurality of links arranged in a single row,

each of said links having a substantially rectangular cross-section and each link includes two parallel fiat sides, a pair of oppositely disposed curved end surfaces perpendicular to said sides, and a pair of opposed edge walls, each end surface extending the full width of the link, and

means for pivotally interconnecting each link to an adjacent link.

2. A mandrel as claimed in claim 1, wherein:

the radius of curvature of said curved end surfaces is smaller than the smallest radius of curvature to which the tube is to be bent.

3. A mandrel as claimed in claim 1, wherein:

said flexible body is pivotally connected to said rigid stem.

4. A mandrel as claimed in claim 1, wherein:

said means for pivotally interconnecting each link to an adjacent link comprises a plurality of circular disks,

said disks and said links being alternately positioned in a row so that each link is separated from an adjacent link by a disk,

each link being pivotally connected to two of said disks.

5. A mandrel as claimed in claim 4, wherein:

each of said links includes a removable flat plate defining a portion of one of said fiat sides,

said plate being received within a complementary recess formed in the body of said link,

said circular disks having raised annular rims formed on each side thereof,

said plate and said link body having complementary arcuate grooves formed therein for receiving the raised annular rims of said disks, and

10 whereby the shoulders define a minimum radius of curvature for the flexible body at which the shoulders of each link will make abutting contact with the shoulders of adjacent links.

References Cited UNITED STATES PATENTS 1/1957 Fuchs 72466 7/1959 Fuchs 72%66 20 LOWELL A. LARSON, Primary Examiner US. Cl. X.R. 

